1. Field of the Invention
The present invention relates to stock packaging materials having a novel adhesive composition pre-applied to the bonding sections thereof and to methods of making such stock materials as well as forming packages from said stock materials, including the filling and closing of said packages.
2. Description of Related Art
Packaging comes in a nearly infinite variety of shapes, sizes and constructions. Though a number of materials are used to manufacture packaging, including glass, metals and metal foils, plastics and cellulosics, the vast majority of packaging is made from plastics and, more commonly, cellulosics, including hybrid materials of cellulosics and plastic materials either as blend or composite material or in laminate form. For more than a century, and particularly within the last half century, industry has sought and, to this day, continues to seek ways to improve packaging, its construction, design and functionality, as well as the methods and processes by which such packaging is formed and/or filled and closed. While much of this development effort has been focused on improving traditional packaging materials and processes, a large portion has also been directed towards the creation of new packaging with new and improved properties and characteristics for addressing three key objectives. The first is the creation of new packaging for old products, e.g. transitioning milk products from glass containers to cellulosic and plastic based containers. The second is the creation of packaging to accommodate new applications/uses for an old product, e.g., transitioning from traditional frozen food containers to ones that can be reheated in a conventional or microwave oven. Finally, the third is the creation of packaging for entirely new products, e.g., microwave popcorn.
The design and construction of a specific package, including its size, shape, type of material from which it is made, and process by which it is manufactured, etc., turns largely on the application to which the package is to be employed. For example, paperboard packaging for mailers like Express Mail, Priority Mail, FedEx and UPS envelopes will have entirely different performance and materials requirements than corrugated bulk containers, e.g., >10 ft3 cartons, intended for use in packaging plastic pellets, chemical additives and the like. Even within a given class of packaging materials, as with paperboard containers, those to be used for processed dry-good foods like cereals, baking mixes, and the like, will have markedly different materials, performance and manufacturing requirements from those to be used for prepared frozen foods that are to be reheated in their packaging in conventional and/or microwave ovens.
As noted, the compositional make-up and structural design of packaging materials will vary depending upon the particular application to which the packaging is to be applied, e.g., corrugated v. paperboard, laminated v. coated, etc.; however, perhaps one of the most critical elements of packaging design and construction is the selection of the adhesive to be employed in the construction and formation of the packaging. Packaging adhesives vary widely in chemistry, formulation, application, activation, properties, characteristics, performance and the like. Selection of the adhesive is critical to the particular end-use of the packaging and, most often, is determinative of the method and apparatus by which the packaging is to be formed. While certain adhesives are relegated to application during the packaging formation and closure operations, others must be pre-applied to the packaging materials for subsequent activation during the packaging formation and closure operations. Still others have the capability of being used in both process methods.
Although natural adhesives such as natural rubber latexes and colloidal suspensions of proteinaceous materials in water once played a large roll in packaging applications, these have been replaced almost entirely by synthetic adhesives. Even the traditional moisture activated adhesives on consumer envelope closure flaps are being replaced more and more with pressure sensitive adhesives. This shift is being driven by economics and, perhaps more importantly, the broad array of properties and characteristics that make synthetic adhesives better suited for use in a number of applications previously addressed by natural adhesives and suitable for applications that were previously unheard of for natural adhesives. One specific benefit of synthetic adhesives is the ability to custom formulate their composition so as to address the particular needs and performance requirements of the end use application to which it is to be applied: thus ensuring optimal performance.
The four key classes of adhesives employed in industrial packaging applications are solvent based adhesives, heat activated adhesives, hot melt adhesives and reactive adhesives. Each of these traditional classes of adhesives has versatility in use and application, yet each has their limitations and problems and all, in one way or another, create problems or difficulties and/or bottlenecks in high-speed industrial packaging formation and/or closure applications. Although new developments in processes and equipment have been able to address many of the problems associated with each particular adhesive, each such development has added additional costs as well as introduced new concerns and issues as set forth below.
Early on, solvent based adhesives represented some of the more widely used and preferred adhesive materials for packaging applications. These adhesive materials typically comprise either an elastomer or rubber-based material in an appropriate organic solvent or a colloidal suspension of a proteinaceous or other inherently tacky material in water. Of these, the organic solvent based adhesives provided faster bonding times due to their higher concentration of the adhesive material in the organic solvent carrier and the much more rapid rate of evaporation of the organic solvent carrier. In essence, solvent based adhesives typically manifest an immediate tack bond, even without complete evaporation of the solvent. However, given the overriding concerns relative to the use of organic solvents from an environmental, health and safety standpoint, as well as the concerns relative to the affect of such solvents on any printing, surface coatings, and the like of such packaging materials, solvent based adhesives have largely gone out of favor.
Though the aqueous based colloidal type adhesives do find some success in packaging applications; their use is limited and not relevant to high-speed, industrial applications. As mentioned, water is too slow to evaporate and will absorb into the packaging material where it may deleteriously affect the structure and/or strength of the packaging materials, especially those wholly or mostly comprised of cellulosic materials. Although the use of radiant heaters, which accelerate the rate of water evaporation, has addressed these disadvantages to some extent, such diffuse heating, particularly with the higher temperatures needed to speed evaporation, has increased the risk of adverse consequences to the packaging materials themselves, especially any coatings, printing, and the like that may have been applied to their surfaces, as well as to the operating components of the assembly apparatus in the immediate area of the radiant heaters.
As is apparent from the foregoing, typical solvent based adhesives are applied in-line and, with the exception of pressure sensitive-type adhesives, are incapable of being pre-applied. Although pressure sensitive adhesives may be pre-applied, they are impractical for industrial packaging formation due to the need to apply a release paper or like material over the pre-applied adhesive to prevent premature bonding. Thus, their use in packaging is essentially limited to use on the closure means of packaging, especially paperboard envelopes intended to be filled and sealed by the consumer: not in a high-speed industrial filling and closing process. Regardless, generally speaking, solvent based adhesives have limited utility and desirability due to their relatively poor adhesive performance and/or strength.
The class of packaging adhesives having the broadest array of chemistries and applications are those known as the heat activated adhesives. Heat activated adhesives belong to two distinct, yet very broad, subclasses of adhesive chemistry, namely thermoplastic and thermoset adhesives. Heat activated adhesives also have the greatest versatility in use as well as application and are capable, generally, of being applied in-line or pre-applied.
The subclass of thermoplastic adhesives embraces a broad spectrum of chemistries including polyethylene and ethylene copolymers, especially ethylene acrylic acid copolymers; polyvinylacetate and vinyl acetate copolymers, especially ethylene vinyl acetate copolymers and vinyl acetate acrylic acid copolymers; polyesters and sulphonated polyesters; and the like. Thermoplastic adhesives are typically applied in the form of a suspension or emulsion of discrete domains of the thermoplastic material in water, oftentimes as a colloidal suspension, although the pre-application of a film by extrusion/co-extrusion is also known. The thermoplastic adhesive may be applied in-line to the intended bond site of the packaging material just prior to or concurrent with the package formation step or, more commonly, pre-applied to the packaging material or blank for subsequent activation during the packaging formation. Pre-application of the thermoplastic adhesive may be to the whole or substantially the whole of the packaging surface or just to the intended bond site. With respect to the former, besides acting as the adhesive or bonding agent, these thermoplastic coatings and films also serve as barriers and protective coatings to the packaging materials and can be further used in providing decorative benefits to the packaging.
Activation of the thermoplastic adhesive is achieved by heating the thermoplastic material to a temperature at or above that at which the thermoplastic becomes tacky and/or melts, typically its glass transition temperature and/or melt temperature. Preferably, bonding is achieved by melting the thermoplastic material so that the liquid melt may wet and/or physically infiltrate the substrate surface(s) of the bond site. In the case of the thermoplastic adhesives applied as a suspension, the heat also drives off the water carrier. The bond itself is formed upon the cooling of the thermoplastic melt.
Although heat activated thermoplastic adhesives have achieved great success; they are not without their limitations. For example, they are not suitable for use in high temperature applications due to the fact that such high temperatures may soften, if not melt, the adhesive leading to a weaker bond or a failed bond altogether. Similarly, these thermoplastic adhesives are not suitable for use in applications where the packaging is to be subject to freezing conditions due to the fact that any trapped water will expand upon freezing, causing the bond to fail. Though this is less of a problem for pre-applied thermoplastic adhesives, for in-line applied adhesives short dwell times in the heat activation stage oftentimes is insufficient to allow for complete evaporation of the water carrier. While the selection of higher melt temperature adhesives and longer heating times may address the foregoing, these introduce new problems. For example, higher temperatures as well as longer dwell times will require longer cool down periods before the bond forms. Thus, the production line must be slowed down, extended to provide a longer dwell time in the cool down cycle and/or modified to introduce accelerated cooling means.
The other key subclass of heat activated adhesives, and one that overcomes many of the limitations and deficiencies of thermoplastic adhesives, is that of the thermoset adhesives. Like thermoplastic adhesives, thermoset adhesives embrace a broad array of chemistries including epoxy resins, novolak resins, polyvinyl butyral resins, acrylic resins, and thermosetting polyester resins. These materials have especially high temperature resistance, much higher than found with typical thermoplastic adhesives, and are particularly suited for use in high temperature packaging applications, especially in the packaging of prepared and frozen foods, most especially for those prepared and frozen foods to be reheated in their packaging in conventional and/or microwave ovens. Like thermoplastic adhesives, thermosetting adhesives can be pre-applied or applied in-line as a liquid; though use as a pre-applied is essentially limited to those thermoset chemistries that are solid and dry-to-the-touch at room temperature and soften/liquefy and then cure or set at higher temperatures. Perhaps the most common of the pre-applied thermoset adhesives are the thermoset polyester resins. These materials may be applied to the intended bond site or, like thermoplastic coatings, as a coating over the whole or substantially the whole of the packing surface, especially the inner surface where it provides excellent high temperature resistance for use in food packaging. In particular, these thermoset coatings provide excellent barrier properties to hot liquids, greases and the like and resist the high temperatures generated during filling, sealing and cooking operations.
While thermoset adhesives have a much higher temperature resistance than the thermoplastics, and, thus, much broader uses, they also have a markedly higher activation temperature, oftentimes two or more times that needed for the thermoplastic adhesives. Indeed, many thermoset adhesives require surface temperatures on the order of 225° C. to 550° C. in order to achieve a suitable bond. Such high temperatures create additional concerns relative to the heating apparatus, the affect of the higher temperatures on the packaging materials as well as the proximate components of the assembly apparatus. Additionally, while thermoplastic packaging adhesives raise concerns relative to the cool down time, thermoset adhesives have long cure or set times to be dealt with. As with the thermoplastic adhesives, longer cure or set times mean slower production speeds and/or more expensive and complicated equipment. Though there are those, such as Baker (U.S. Pat. No. 4,249,978), who apply a fine mist of a colloidal suspension of a thermoplastic adhesive over the thermoset film or coating so that a tack bond may be formed while the thermoset material sets up or cures, such a process introduces weaknesses in the bondline and reintroduces concerns relative to trapped water.
However, issues and concerns relative to the limitations and problems associated with a given heat activated packaging adhesive are not the only issues and concerns befalling packaging and packaging processes. Indeed, the activation of such adhesives, or more precisely the method and means of activation, introduces many additional and, perhaps, greater concerns.
Early on, activation of heat activated adhesives was dependent upon radiant heating, especially radiant heating generated by quartz lamps and RF induction. However, radiant heat had many limitations including the maximum temperatures attainable and the length of time needed to attain the activation temperature needed for the chosen adhesive. Additionally, the equipment was such that it was difficult, if not impossible, to localize the heat and, consequently, prevent adverse effects on the packaging materials themselves, particularly in areas away from the bond site, as well as on various elements of the assembly line that were also exposed to the radiant energy. For example, radiant heating also resulted in the melting of thermoplastic materials near, but removed from, the intended bond site. Similarly, components of the assembly or manufacturing apparatus within the field of the radiant heat would also heat up and, consequently, shorten their life or maintenance cycles. Furthermore, radiant heating was costly, consuming high levels of energy due to low heat generation and poor utilization efficiencies.
A major evolution in packaging technology, and one driven, at least in part, by the advent of and advances in thermoplastic and thermoset adhesives, was the transition from radiant heating to localized, directed heating through the use of hot air streams, most typically as applied through one or more or a series of directed nozzles. While these directed nozzles addressed many of the concerns with radiant heating, especially the temperatures attainable and the speed with which those temperatures could be attained, they failed, in whole or in part, to address other concerns such as the impact on the packaging and the elements of the assembly line. Indeed, because higher temperatures were now being generated, even more concern arose relative to charring or deleterious effects on the packaging, especially coatings and printing thereon, as well as of the handling equipment itself. Specifically, oftentimes these heater nozzles were fixed and, when the assembly line stopped or there was a gap between packaging components on the assembly line, high heat built up in the packaging or the components of the assembly line, especially the conveyor belts and associated mechanical components. With the former, charring and, possibly, fire could result. In the latter, the high temperatures shortened the life of the conveyor belts as well as degraded the lubricants in the equipment, thus shortening their life or the cycle time between repairs and maintenance. Although these issues could be addressed by turning off the heaters, such action resulted in longer delays between shut down and restart of the assembly line to enable the heater to reach its activation temperature.
Eventually, many of these concerns were partially addressed by the subsequent development of hot air heaters that retracted or moved away from the surface of the conveyor when there was a large gap between successive packaging materials or in the event the assembly line itself were stopped. Although such retraction means moved the hot air stream away from the packaging materials, it oftentimes redirected the stream of hot air to the conveyor belts and other components of the apparatus itself. Thus, while one problem was being addressed, another arose or, if pre-existing, was oftentimes exacerbated or more pronounced.
Nevertheless, additional advances were still being made to address these new concerns. One especially successful advance was the incorporation of cooling means into the apparatus in the same region as the heater nozzles to draw heat away from the packaging material and to cool the hot air streams as they passed from the adhesive materials to which they were directed. A second advance was the incorporation of shielding and/or exhaust means that redirected and/or captured, respectively, the hot air stream as it passed from the adhesive material so that it could not affect the apparatus or the packaging materials. Though each certainly aided in addressing the concern with the hot air streams, they did not completely address the matter. For that reason, Landrum et. al. (U.S. Pat. No. 5,562,795), among others, employed aspects of both cooling and exhaust to maximize relief from and minimize any deleterious effects of the hot air streams.
While the advances in packaging formation and closure technology and heat activated adhesive technology have synergistically paralleled one another; it is evident that each advancement brought with it new challenges and issues as well as the improvements and benefits thereof. Though the latest technologies have addressed many of the outstanding issues and concerns, they are not entirely alleviated. For example, while the dual adhesive systems of Baker ('978), as mentioned above, did a lot to address the bottleneck created by faster activation achieved with the hot air stream nozzles, their application introduced new issues. Specifically, although a majority of the fine mist will deposit where intended, there is always some percentage that wafts off, eventually depositing on other sections of the package or, worse, the equipment where it builds up over time to cause problems necessitating shut down for cleaning and/or repair.
Perhaps the greatest achievement of these advances in both adhesive chemistry and application/activation means was the concurrent and marked increase in line speeds, enabling greater output over a given time. Indeed, although individual developments have, on occasion, caused a retraction or loss of line speed, the overall trend has been a marked increase going from, for example, the 8.25 second per box production time so proudly touted by Gobalet in 1958 (U.S. Pat. No. 2,984,598) to the 150 to 200 carton per minute production rate of Landrum et. al. in 1995 (U.S. Pat. No. 5,562,795). While these advances have found great success in providing more secure boxes faster, each successive generation and advancement has also added new complexity to the package formation and closing apparatus, more and expensive components that could be new problem areas, and additional spatial needs to accommodate ever-increasing assembly line apparatus to address bottlenecks: all of which have markedly increased the costs and commitment of capital resources to said packaging operations.
Furthermore, despite all the benefits and attributes of the advanced heat activated adhesives and their activation systems; they are not a panacea for the packaging industry. Although their high temperatures and thin film adhesives allow for quick activation with little heating of the underlying packaging, these thin films of adhesive, whether in a pre-applied state or as applied in-line, have essentially no profile or thickness and, therefore cannot accommodate gaps of any note resulting from surface variations in the packaging surfaces to be mated. Indeed, as seen in Heinz (U.S. Pat. No. 5,632,712), their use in closing packaging having shied flaps requires specialized closing apparatus in order to apply uniform pressure across the whole of the flap or bond site so as to accommodate the rise where the one flap overrides the end of the other. However, this specialized apparatus will not address surface variations in the flaps themselves. For example, the surface of corrugated cardboard oftentimes has a series of ridges and valleys corresponding to the underlying corrugation in the cardboard. While a thin film of thermoplastic or thermoset adhesive would allow for a contact bond at intersecting ridges, the thickness of the adhesive would most often be insufficient to span the gap between opposing valleys; thus, resulting in weak bonds. Efforts to increase the thickness of the adhesive film will only slow down line speed or necessitate longer heating sections of the assembly line in order to allow sufficient heating to ensure complete activation or melting of the thicker adhesive. The resultant longer residence time in the heating step will also markedly increase the likelihood that the surface of the packaging will itself be heated to an adverse temperature. Similarly, since more adhesive is present, the cooling period in the case of thermoplastic adhesives or the cure or set period in the case of thermoset adhesives will be longer which means that the residence time in the mating and bonding step will be lengthened as well. Thus, while certainly critical to the packaging industry, traditional heat activated thermoplastic and thermoset adhesives are not entirely without their shortcomings.
Finally, yet another subclass of heat activated, pre-applied thermoplastic adhesives are those known as reactivatable adhesives. Such adhesives are more clearly described in, for example, Gong et. al. (US 2003/0041963 A1; US 2004/0164134 A1; US 2004/0164135 A1 and US 2004/0166309 A1); Nowicki et. al. (US 2004/0163754 A1; US 2004/0163768 A1; and US 2004/0166238 A1) and Pierce et. al. (US 2004/0166257 A1), among others. In their simplest of embodiments these reactivatable adhesives are pre-applied thermoplastic materials, especially hot melt type materials as further described below, which are activated by an induced or internally or proximately generated heat. In essence these reactivatable adhesives are reactivated by exposing the same to sufficient energies, typically in the form of ultrasound, near infrared radiation (NIR) or electromagnetic energies, to melt the pre-applied material. Induction reactivation requires the presence of susceptors in or next to the adhesive material: thus, introducing new elements into the adhesive and/or the packaging construction. Although these adhesives and reactivation systems avoid the problems associated with hot air nozzles, the reactivation processes introduced a number of new problems, particularly health and safety problems, since one needs to protect the workers and apparatus from the deleterious effects of, e.g., ultrasound and NIR. Also, each of these adds new costs with respect to the ultrasound and NIR generating equipment and associated protective equipment needed.
Furthermore, reactivation processes appear to adversely impact the line speeds that may be realized and are limited with respect to the types of apparatus and package forming systems with which they may be used. For example, Nowicki et. al. (US 2004/0163768 A1) teaches that reactivation will take place within 10 seconds, preferably less than 5 seconds, most preferably less than 3 seconds, of its exposure to the reactivation energy and that a suitable bond will thereafter be formed upon compression for less than 30 seconds, more preferably less than 15 seconds. Even if one is able to achieve the preferred rates, these methods represent a tremendous loss in production rate, especially as compared to the 150-200 per minute carton production rate claimed by Landrum et. al. (U.S. Pat. No. 5,562,795), which uses hot air activation, as discussed above.
The third key class of packaging adhesives, and clearly, from a volume standpoint, the most successful of adhesive technologies in the packaging industry, is the hot melt adhesives. Though generally thermoplastic, hot melt adhesives are characterized as involving the in-line application of a bead of a melt of a thermoplastic adhesive material to the bondline prior to mating of the surfaces to be boned. Like other adhesives, the class of hot melt adhesive materials embrace a wide array of chemistries, e.g., polyethylene and ethylene copolymers, polyvinylacetates and vinylacetate copolymers, polyamides and the like, each having different performance and property profiles. Their versatility in performance as well as their relative ease of applicability has made them a favored choice in many packaging applications. For example, their relative viscous state combined with the ability to adjust the die of the nozzle head from which the hot melt material is exuded allows for controlled variation of the bead of adhesive dispensed, facilitating conformation to and accommodation of surface variations in the packaging materials, particularly gaps at the bond site, as well as various bond width and the like.
Despite all their attributes, the hot melt adhesives and packing methodologies employing the same are not without their problems, including relatively low heat resistance. Even though reactive hot melts will have improved high temperature performance, they still do not achieve the high heat resistance of many of the thermosetting materials, thus making them inappropriate for food packaging intended to be reheated in microwave or conventional ovens. Furthermore, the equipment needed for maintaining and dispensing the hot melt adhesive is expensive and not without its problems as well. For example, long residence times in the holding tank/melt chamber may lead to degradation in performance of the hot melt adhesive and/or, in the case of reactive hot melts, premature cross-linking. Additionally, the dispenser nozzles have a tendency to clog, particularly as a result of the presence of solid particles such as dirt, debris and other contaminants that may be present in the hot melt or enter the tank or chamber in which the molten hot melt is held prior to dispensing. Such clogging may also occur as a result of temporary stoppages in the assembly/production line or in the event of large gaps between packages on a given line whereby dispensing is stopped for a sufficient period of time to allow the hot melt on the nozzle tip or die to cool. Clogging becomes of particular concern with reactive hot melts where one may have to shut down and clean out the whole hot melt dispenser apparatus before restarting. Regardless, any shut down of a high speed packaging and filling line, even one of relatively short duration and for such a seemingly innocuous process as a cereal box filling and closing operation, may cost hundreds of thousands of dollars in lost production revenue.
The use of hot melt adhesives also has a substantial capital cost in initially setting up such a production line which also necessitates a lengthy cool down section since hot melts are traditionally applied in thicker amounts than typical heat activated adhesives: thus necessitating longer cool down period for the bond to form. Additionally, hot melts and their dispensing equipment also present a number of concerns from a health and safety standpoint due to the large dispensing equipment involved, which is maintained at temperatures above the melt temperature of the hot melts. Workers attempting to clean the hot melt dispensing equipment or other proximate equipment during routine maintenance or in the event of a problem on the production line are exposed to the hot dispensers, as well as the molten hot melt. While the heaters could be turned off and allowed to cool, such an event would mean that the hot melt contained in the equipment will also cool down and, thus, start up of the production line will be further delayed in order to bring the hot melt back to dispensing temperature.
Notwithstanding the foregoing, perhaps one of the most persistent problems with the use of hot melts is the stringing of the hot melt adhesive after dispensing is stopped. These strings or drools of the hot melt fall on other areas of the packaging materials and, worse, the assembly line itself and its integrated equipment. While the former may lead to rejected parts, which can easily be identified and discarded, the latter may lead to line shut downs to allow for proper cleaning. Efforts have been put forth to address these issues, at least in part. For example, Baker (U.S. Pat. No. 3,511,138) employed a specialized advancing and retracting dispenser so that the string of adhesive falls back upon the previously applied adhesive as the dispenser is retracted. This, however, is not suited for a continuous, especially a high-speed continuous, assembly line operation.
The fourth class of packaging adhesives is that know as the reactive adhesives. These comprise one and two-part curable adhesive systems that cure under ambient conditions: though heat may accelerate their cure. Like the aforementioned classes of packaging adhesives, these too include a broad array of chemistries including, for example, acrylic esters, polyurethanes, phenol formaldehydes, cyanoacrylates, and the like. For the most part, reactive adhesives are applied in-line as “100% solids” liquid, i.e., they do not have a non-reactive liquid carrier or solvent like the solvent based adhesives; rather, the curable components are themselves liquid or are soluble in one or more of the co-reactive components of the adhesive. These adhesives typically are found in three forms, one part systems, multi part systems (most often two-part systems) and encapsulated systems.
Cure or setting of one-part reactive adhesive is often slow due to the fact that they rely upon an environmental condition to effectuate cure. For example, those that rely upon exposure to moisture require long open times before the surfaces to be bonded may be mated. Additionally, the fact that an adhesive wetted surface is open presents and opportunity for foreign matter to fall upon the adhesive wetted surfaces which foreign matter can interfere with the bond and/or result in a commercially unacceptable product. Adhesive systems that rely upon anaerobic conditions, while able to be mated immediately, cure very slowly due to residual oxygen inhibition. Besides their slow cure, these reactive adhesives tend to be of fairly low viscosity such that the adhesive material may run out of the bond interface or be absorbed and/or wicked into the packaging substrate. Consequently, there may be insufficient adhesive material at the bondline to effectuate a good bond.
Two-part adhesive systems are better suited for industrial applications as their cure speed is or can be made much faster. However, these adhesive systems require expensive and complex dispensing equipment that mixes the two parts immediately prior to or concurrent with dispensing of the same. Here, because viscosity builds once cure is initiated, run out and wicking is less of a problem; however, great concern arises in the event of a temporary stoppage of the production line, even for a few seconds, as cure of the adhesive composition will occur in the dispensing equipment. Unlike hot melts that can be readily removed by reheating the hot melt, these reactive adhesives are thermoset in nature and cleaning of the apparatus, if cleanable, is time consuming. Thus, the risks associated with such two-part dispensing systems make them in appropriate or undesirable for high speed industrial packaging applications.
In order to address many of the problems associated with the aforementioned reactive adhesives while retaining the faster cure speeds capable with such systems, the industry has employed pre-applied encapsulated adhesives, albeit to a very limited extent. Typically these encapsulated adhesives comprise a plurality of microcapsules containing liquid curable adhesive materials with at least one of the primary activators or curatives for effecting polymerization or cure being incorporated into different microcapsules or into the binder material which holds the microcapsules to the surface to which they are applied. Cure is initiated by fracturing the microcapsules so as to allow the reactive components to intermix and react. Fracturing is typically accomplished by first mating the two surfaces to be bonded, one of which has the encapsulated adhesive pre-applied to its mating surface, and then subjecting the area of the intended bond to compressive forces, such as by passing the mated surfaces through one or more pinch rollers or under a stationary blade or by manually scraping the mated bondline area with, e.g., a coin edge, a razor or straight edge, etc. The compressive forces fracture the microcapsules, thereby releasing and/or enabling the intermixing of the curative with the remainder of the reactive components, and create a flow of the liquid components whereby the components of the liquid curable adhesive composition are intermixed with the curative and cure, and thus bonding, is effectuated.
Although encapsulated adhesives have found great success in the assembly of machinery and the like, especially as thread locking materials, they have found very little and very limited use in packaging. Essentially, their use has been limited to paper bonding applications, especially in the production and/or closure of paper envelopes for letters, junk mail and the like, as shown in Akridge et. al. (U.S. Pat. No. 5,794,409) and Haugwitz (U.S. Pat. No. 4,961,811). Even here, their use is limited and not optimal despite their traditionally strong bond due to a number of factors including the low viscosity of the liquid curable components combined with the porosity of typical packaging materials. As noted above, wicking of the liquid curable components into the packaging substrate leaves very little curable material in the bond gap or interface to create the bond. This is not such a problem with the thinner paper and very planar surfaces found with envelopes where the liquid materials often saturate the surface layer of the paper, which saturation provides sufficient adhesive material to effectuate the bond. However, a different result is found with thicker packaging materials, such as paperboard and especially cardboard, where the liquids often absorb or wick deep into the subsurface, leaving little liquid curable material at the interface, and certainly an insufficient amount to address surface irregularities often found with these materials.
Another factor limiting the use of encapsulated adhesive in packaging applications, other than paper envelopes, is the inability to provide sufficient compressive forces to ensure good microcapsule fracture. Most packaging materials tend to have good absorption of compressive forces. This is especially true for thicker paperboard and corrugated cardboard. The absorption or cushioning of the compressive forces leads to poor and/or insufficient fracturing of the microcapsules; thus, reducing the amount of liquid curable materials released at the bond interface to cure. Although higher compressive forces could be used to increase microcapsule fracture and flow characteristics, such forces will likely have an adverse effect, especially disfigurement, on the surface appearance of the packaging.
Notwithstanding the foregoing, perhaps one of the most telling of limitations for the use of encapsulated adhesives in packaging is the inability to even apply compressive forces to the mated surfaces due to package design and construction. Other than envelopes, as mentioned above, unless the packaging has flanges or is tube-like, both of which support crimping, there is little opportunity to provide the requisite support or back pressure to the underlying surface to ensure sufficient compression at the bondline. For example, if one were endeavoring to bond opposing top flaps to a cereal box using an encapsulated adhesive, compressive forces needed to fracture the encapsulated adhesive would like lead to the collapse of the box, with both flaps being pushed into the interior of the box, absent a backstop or counter force. Furthermore, because of the fragility of the microcapsules, especially in order to ensure the presence of sufficient liquid curable material to create the bond, concern also arises with respect to the premature and/or unintended fracturing of the microcapsules due to rough handling, stacking, processing and the like, especially as may be found in high volume, high speed industrial applications. Thus, while traditional encapsulated adhesives and their method of activation would appear, in retrospect, to offer a solution to many of the issues found with traditional packaging adhesives, they too have their limitations and appear inappropriate for use with other than thin paper packaging such as envelopes.
While each of the aforementioned prior art adhesive systems and their respective methods of application and activation, if appropriate, have found their niches in the packaging industry, none may be considered as or be considered to approach that of a universal adhesive system for the packaging industry. Those of the prior art systems that seem to offer the broadest performance characteristics and versatility, especially the heat activated systems and the hot melts, have one common element, each requires heat, especially high heat, to apply and/or activate/reactivate the adhesive composition. As noted, the use of heat, though necessary, has a number of adverse or potentially adverse consequences from a health and safety standpoint, particularly with respect to potential exposure of line workers to the heat generating means, the hot air streams, etc. during normal operation as well as in addressing potential problems in those areas of the assembly and filling lines where they are employed. Additionally, such high temperatures, especially if the heat is misdirected, may adversely affect the packaging materials themselves as well as the package forming and closing apparatus. The former is especially of concern with respect to any coatings, especially varnishes, and print or graphics that may be applied to the surfaces thereof, as well as the packaging substrate itself, especially in the case of an assembly line stoppage which may lead to charring and burning. The latter is of concern where there are large gaps between packaging blanks on the assembly line or in the event of an assembly line stoppage. Consequently, in addition to the high energy costs associated with merely generating the heat as well as the costly heat generation equipment itself, additional expense is incurred in installing appropriate safety equipment for shielding the equipment and personnel, for dissipating and/or exhausting the heat, and/or for added maintenance on the apparatus, as appropriate.
Thus, there exists a need in the packaging industry for an adhesive system that, if not universally applicable, is applicable to a broad spectrum of packaging substrates, designs and applications. In following, there exists a need in the packaging industry for such an adhesive system that does not require heat for activation/reactivation or application but which is fast curing and is capable of forming a bond within fractions of a second. Furthermore, there exists a need in the industry for an adhesive system that can be pre-applied to packaging stock materials and blanks, thereby removing the adhesive application from the forming, filling and closing operations.
In following with the foregoing, there exists a need for packaging stock materials, including packaging blanks, having pre-applied to the bonding surfaces thereof an adhesive system that addresses the aforementioned needs.
There also exists a need for such packaging stock materials wherein the pre-applied adhesive material is such that it is not susceptible to premature activation or release upon exposure to high temperatures or moderate forces experienced in the work, storage or transport environment, including as a result of stacking or rough handling associated with high speed industrial packaging formation and filling operations.
In addition, there is a need in the packaging industry and the packaging industry would be greatly benefited from a package forming and/or closing process which avoids the need for the application of an adhesive, particularly liquid or molten adhesives, and relies instead on the use of packaging stock materials having pre-applied thereto a dry to the touch, fast curing adhesive material.
Further, the packaging industry would be much benefited from packaging stock materials having a pre-applied adhesive thereon which adhesive does not require the use of heat, whether by direct application of heat or indirect or induced, as with radiation (especially NIR) or ultrasonic energy, to activate or reactivate the pre-applied adhesive material. In following, it would be desirable to have a high speed industrial package formation, filling and closing process which does not suffer bottlenecks as a result of open times, cure time or cool down times needed to effectuate a proper cure and/or achieve a suitable tack bond with the packaging adhesive.
Further, there is a need in the packaging industry for a package forming, filling and closing process wherein the bonding steps for the package formation and/or closing are near instantaneous, if not instantaneous, at room temperature.
In addition, the packaging industry is in need of and would be greatly benefited by a packaging formation, filling and/or closing apparatus which is simplified and avoids the need for specialized equipment for generating and applying heat to heat activated/reactivated adhesives or for applying a molten hot melt adhesive, which eliminates the need for protective equipment and apparatus for protecting the assembly line, the operators thereof as well as the packaging materials themselves, and which allows for instantaneous or near instantaneous bonding without need for heat up and cool down cycles, the latter of which especially adds to the length of the assembly line and complexity of the packaging apparatus.
Finally, it would be especially desirable and the packaging industry would be especially benefited from a packaging forming and/or closure apparatus which addresses the many problems and shortcoming associated with current packaging adhesives and associated packaging formation apparatus and does so in a way that is no more expensive and/or capital intensive than traditional methods and does not adversely affect the through put speed of such a process. In particular there is a need in the packaging industry for a packaging process which is less expense and/or capital intensive; faster with less risk of bottlenecks, line stoppages, etc., and more versatile in terms of applications, substrates, formation and closure processes and the like.
Accordingly, it is a primary objective of the present invention to provide packaging stock materials, including blanks, that have a pre-applied adhesive which adhesive and stock materials overcome or significantly address all or most all of the aforementioned problems and concerns associated with the prior art packaging adhesives, stock materials and processes. It is also a primary objective of the present invention to provide a packaging formation, filling and/or closing process which employs such packaging stock materials and which overcomes many of the problems associated with the prior art processes. It is also an object of the present invention to provide a packaging forming apparatus which does not require the use of heat, direct or induced, for curing and/or activating an adhesive in the package formation and closing operations and, if heat is employed, such heating is merely ancillary to the cure or polymerization process and of relatively low temperature and limited duration. Finally, it is an object of the present invention to provide a packaging forming apparatus which eliminates the complicated and oftentimes lengthy equipment associated with current packaging apparatus.